A multiple-disc clutch uses a series of annularly shaped friction discs to transmit torque or apply braking force. The discs have internal teeth that are sized and shaped to mesh with splines on the clutch assembly hub. In turn, this hub is connected to a planetary gear train component so gearset members receive the desired braking or transfer force when the clutch is applied or released. The multi-disc clutch assembly is the means by which gears within the transmission are selectively engaged and disengaged from either the crankshaft or the transmission case.
Multiple-disc clutches have a large drum-shaped housing which holds all other clutch components, such as the cylinder, hub, clutch piston, piston return springs, seals, pressure plate, clutch pack and snap rings. The clutch pack consists of a series of clutch plates and composition plates, and a thicker plate known as the pressure plate or clutch backing plate. Sometimes friction discs are sandwiched between the clutch plates and the pressure plate.
The clutch plates have tabs extending radially outwardly around their outside diameter to mate with the axially aligned channels or grooves formed in the inner rim of the clutch drum. When the tabs are held within the grooves, they prevent substantial relative movement between the clutch plates and the clutch drum. Clutch plates must have perfectly flat outer surfaces. They are specifically machined to promote a coefficient of friction to help transmit engine torque.
The pressure plate or clutch backing plate has tabs extending radially outwardly around the outside diameter also to mate with the channels in the clutch drum. The pressure plate is usually held in place with a large snap ring. The stroking piston forces the engaging clutch pack against the fixed pressure plate. Because the pressure plate cannot move or deflect, it provides the reaction to the engaging clutch pack.
Examples of prior art multi-disc clutch assemblies are shown in the following patents: U.S. Pat. No. 4,958,753; U.S. Pat. No. 4,592,455; and U.S. Pat. No. 4,226,319.
In addition, two examples of prior art multi-disc clutch assemblies are shown in FIGS. 1-4. In the prior art GENERAL MOTORS.RTM. THM 400 automatic transmission, illustrated in attached FIGS. 1 and 2, a clutch piston 46 is provided between the inner surface of the clutch drum housing 12 and the composition plates 32 and flat steel clutch plates 34 of the clutch pack in the clutch drum assembly 10. A waved steel ring 24 is provided between the clutch piston 46 and the bottom clutch plate 34 or composition plate 32. The ring 24 defines an axial opening that fits around the inner rim 18 of the clutch drum housing 12 and has radially outwardly extending tabs or projections 26 that fit within the axial grooves 20 defined in the inner surface of the outer rim 16 of the clutch drum housing 12. The waved steel ring 24 further defines axially downwardly extending portions 28 and axially upwardly extending portions 30. Clutch backing plate 38 (sometimes also called the pressure plate) nests against one of the clutch plates 34 and is held in place by snap ring 44. Clutch backing plate 38 defines an axial opening that fits around the inner rim 18 of the clutch drum housing 12, and the plate 38 has radially outwardly extending tabs or projections 42 that fit within the axial grooves 20 defined in the inner surface of the outer rim 16 of the clutch drum housing 12.
When an axially directed pressure of a sufficient force (due to the stroking piston) is applied against the waved steel ring 24, the downward and upward projecting portions 28, 30 of the ring 24 may be flattened out to cause a greater frictional engagement between the surfaces of the ring 24 and the surface of the adjacent composition plate 32 or clutch plate 34 and the surface of the clutch piston 46. The reacting forces of the ring 24 cause the clutch to apply more gradually to prevent a bump or lurch at the end of a gear shift. Because the tabs 26 of the waved steel ring 24 are held within the axially extending grooves 20 of the clutch drum housing, the waved steel ring 24 cannot rotate when subjected to the axial force in combination with the rotational frictional forces.
Another prior art clutch arrangement is shown in FIGS. 3 and 4. The ACURA.RTM. INTEGRA.RTM. 4Sp Hydraulic transmission includes a clutch drum assembly 50 having a clutch housing or drum 52 defining an annular open cavity 54 between an outer rim 56 and an inner rim 58. Axially extending grooves 60, sometimes called clutch plate channels, are formed in the inner surface 62 of the outer rim 56. A clutch piston 66 is held within the open cavity defined by the clutch housing 52. The clutch piston 66 has an axial opening 68 that fits around the inner rim 58 of the clutch housing 52, and has an annular groove 70 disposed in its upper surface. A spring washer 64 having an inner edge, an outer edge, an upper surface and a lower surface, is held within the annular groove 70 with a portion of its lower surface contacting the clutch piston. One clutch plate 72 of a series of clutch and composition plates 72, 76 forming the clutch pack abuts against a portion of the upper surface of the spring washer 64. Each of the clutch plates 72 has radially outwardly extending tabs 74 that fit within the axially extending grooves 60 in the inner surface 62 of the outer rim 56 of the clutch housing 52. The composition plates 76 have radially inwardly extending teeth 78 that selectively mate with splines provided on the transmission shaft (not shown). A clutch backing plate 80 (sometimes called a pressure plate) nests adjacent to the last clutch plate 72 of the clutch pack, and is held in place by snap ring 90. The clutch backing plate 80 has an annular open cavity 82 that fits around the inner rim 58 of the clutch housing 52, and has radially outwardly extending tabs 84 that fit within the axially extending grooves 60 in the inner surface of the outer rim 56 of the clutch housing 52.
The spring washer 64 is inwardly dished so that its upper surface slopes downwardly from its outer edge 65 to its inner edge 63. Typically, the spring washer 64 is inserted into the annular groove 70 so that the upper surface is adjacent to a clutch plate 72 and the inner edge and a portion of the lower surface contact the annular groove 70. The spring washer 64 opposes axial forces exerted on the clutch piston 66 (due to the stroking piston). When those forces are high enough, however, the spring washer 64 may be flattened out within the annular groove 70 of the clutch piston 66. It has been found that frictional contact between outer edge 65 of the spring washer 64 and the facing surface of the adjacent clutch plate 72 cuts a groove or otherwise causes excessive wear in the adjacent clutch plate 72. This wear causes a small loss of the axial spring force applied to the clutch plates, which in turn causes undesirable slippage between the clutch piston and clutch pack, eventually leading to clutch failure.
Alternately, the spring washer 64 may fit within groove 70 in an inverted position such that the outer edge 65 and a portion of the nominal lower surface contact the annular groove 70 and the inner edge 63 and nominal upper surface contact the adjacent clutch plate 72. In this configuration, frictional contact between the inner edge 63 of the spring washer 64 and the facing surface of the adjacent clutch plate 72 cuts a groove or wears the surface of the clutch plate 72.
An object of the present invention is to provide a clutch assembly in which the proper axial forces are exerted on the clutch plates and clutch piston without causing undesirable excessive wear on the surfaces of the clutch plates. A further object of the invention is to provide a waved ring spring formed without mating tabs or projections that is held within the clutch piston in the clutch housing without restriction on its rotational movement within the clutch housing.